One problem associated with an outdoor tank resting on a concrete pedestal, typically a large volume tank resting on a concrete pedestal as often found in oil refinery plants, is that rainwater enters the boundary between the bottom side of the tank and the concrete pedestal. The problem is addressed by covering the boundary between the tank bottom and the concrete pedestal with a butyl-based pressure-sensitive adhesive rubber sheet for preventing the entry of rainwater. However, since the butyl-based PSA rubber sheet is poor in weather resistance, heat resistance and freeze resistance, it fails to prevent the entry of rainwater over a long period of time, allowing rust generation at the bottom of the outdoor tank, with potential tank failure.